Speed up my Rendertimes in cycles
Your expectations may be too high for a 4k image.
Your benchmark of 1 minute for the BMW is very good. Note, however, that the image is rendered with only 35 samples and at 50% size. If you increase the size to 100% the render time will quadruple! Increase the size to 400% and it will be multiplied by 16 again. That's 64 times longer for 400% than if the size was 50%. This turns one minute render time into one hour!
On the other hand, your scene is an indoor scene. Such interior scenes need much more samples to get rid of the noise. That's because the light only comes through the window or door and bounces in the room. More samples increase the render time. Light portals can help a bit (What does Light Portal mean?).
For preview in high-res you can render only a part of the image with a box selection, press Ctrl+B. (To remove the box press Ctrl+Alt+B)
Last but not least you can render nice interior scenes with Eevee, too. Have a look here: (youtube) How to make Realtime Archviz with Blender and Eevee (Part 1 of 3)
18 Ways to Speed Up Blender Cycles Rendering (It's from 2017 but gives you an idea about render times and how to save time)